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United States Patent |
6,195,864
|
Chesnes
|
March 6, 2001
|
Cobalt-base composition and method for diffusion braze repair of superalloy
articles
Abstract
An improved cobalt-base braze alloy composition and method for diffusion
brazing are provided for use in repairing superalloy articles, such as gas
turbine engines, power generation turbines, refinery equipment, and heat
exchangers. The improved cobalt-base braze alloy composition includes
nickel; at least one element selected from the group of rhenium,
palladium, platinum, ruthenium, and iridium; at least one element selected
from the group of boron and silicon; and the remaining balance consists of
cobalt. This composition may also include aluminum and/or one or more rare
earth/lanthanide series elements, and the composition may be combined with
one or more powdered base metal superalloy compositions to form an
improved diffusion braze alloy mixture. In the improved method for
repairing superalloy articles, the foregoing mixture is applied to a
region of the superalloy article to be repaired. The mixture is then
heated to melt the cobalt-base braze alloy, thereby joining the base metal
superalloy powder particles together, and joining the entire mixture to
the region being repaired. The molten mixture is next subjected to a
diffusion braze heat treatment cycle in order to break down undesirable
boride and silicide phases and to diffuse the melting point depressants
into the mixture. In a preferred embodiment, the long term diffusion heat
treatment cycle consists of heating the repaired article to 2000.degree.
F., holding that temperature for 2 hours, heating the repaired article to
2100.degree. F., holding that temperature for 22 hours, and cooling the
article to 250.degree. F. After cooling, an environmental coating is
applied to the final repair composite, and this composite significantly
improves the cyclic oxidation resistance of the coating compared to the
properties of the superalloy base metal.
Inventors:
|
Chesnes; Richard Patrick (Zionsville, IN)
|
Assignee:
|
Allison Engine Company, Inc. (Indianapolis, IN)
|
Appl. No.:
|
306968 |
Filed:
|
May 7, 1999 |
Current U.S. Class: |
29/402.01; 29/402.18; 148/425; 228/262.31; 420/438 |
Intern'l Class: |
B23P 006/00 |
Field of Search: |
29/402.01,402.18
228/262.31
420/438,437,436,439,435
148/408,425
|
References Cited
U.S. Patent Documents
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| |
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| |
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|
4339509 | Jul., 1982 | Dardi et al.
| |
4381944 | May., 1983 | Smith, Jr. et al. | 75/255.
|
4478638 | Oct., 1984 | Smith, Jr. et al. | 75/255.
|
4737205 | Apr., 1988 | Selman et al.
| |
4830934 | May., 1989 | Ferrigno et al.
| |
4837389 | Jun., 1989 | Shankar et al. | 428/668.
|
4910098 | Mar., 1990 | Lee et al. | 428/680.
|
4940566 | Jul., 1990 | Wood et al. | 420/443.
|
5066459 | Nov., 1991 | Beltran et al.
| |
5142778 | Sep., 1992 | Smolinski et al. | 29/889.
|
5154885 | Oct., 1992 | Czech et al.
| |
5182080 | Jan., 1993 | Beltran et al. | 420/588.
|
5240491 | Aug., 1993 | Budinger et al.
| |
5273712 | Dec., 1993 | Czech et al.
| |
5282946 | Feb., 1994 | Kinoshita et al.
| |
5320690 | Jun., 1994 | Beltran et al.
| |
5422072 | Jun., 1995 | Mitsuhashi et al.
| |
5455119 | Oct., 1995 | Taylor et al.
| |
5549767 | Aug., 1996 | Pietruska et al. | 148/512.
|
5582635 | Dec., 1996 | Czech et al.
| |
5666643 | Sep., 1997 | Chesnes et al. | 428/549.
|
5806751 | Sep., 1998 | Schaefer et al. | 228/119.
|
5898994 | May., 1999 | Miller et al. | 29/889.
|
5916518 | Jun., 1999 | Chesnes | 420/438.
|
5952042 | Sep., 1999 | Rafferty et al. | 427/189.
|
6077615 | Jun., 2000 | Yada et al. | 428/544.
|
Primary Examiner: Echols; P. W.
Assistant Examiner: Omgba; Essama
Attorney, Agent or Firm: Woodard, Emhardt, Naughton, Moriarty & McNett
Parent Case Text
REFERENCE TO RELATED APPLICATIONS
The present application is a continuation-in-part of a U.S. patent
application Ser. No. 08/827,723, filed Apr. 8, 1997 now U.S. Pat. No.
5,916,518 entitled COBALT-BASE COMPOSITION, which is hereby incorporated
by reference in its entirety.
Claims
What is claimed is:
1. A method of repairing a damaged region of a corrosion resistant
superalloy substrate comprising:
preparing a repair mixture comprising a cobalt-base braze alloy, a base
metal alloy composition and an organic binder, said cobalt-base braze
alloy consisting essentially of, in percent by weight:
Nickel from about 0.001% to less than the weight percent of cobalt;
at least one element selected from the group consisting of:
Rhenium from about 0.001% to about 15%,
Palladium from about 0.001% to about 40%,
Platinum from about 0.001% to about 40%,
Iridium from about 0.001% to about 12%, and
Ruthenium from about 0.001% to about 12%;
at least one element selected from the group consisting of:
Boron from about 0.001% to about 6% and
Silicon from about 0.001% to about 10%; and
at least one element from the group consisting of:
Chromium, Aluminum, Titanium, Tungsten, Molybdenium, Niobium, Rhenium,
Hafnium, Tantalum, Iron, Manganese, Rare earth/Lanthanide elements,
Carbon, and Zirconium; and
the balance Cobalt;
depositing said repair mixture on at least a portion of the damaged region;
and
joining the deposited repair mixture to the superalloy substrate.
2. The method of claim 1, wherein said joining includes a long term
diffusion heat treatment.
3. The method of claim 2, wherein the long term diffusion heat treatment
includes:
heating said depositing repair mixture to a first temperature between about
2000.degree. F. and about 2100.degree. F.;
heating the depositing repair mixture to a second temperature greater than
or equal to about 2100.degree. F.;
maintaining the deposited repair mixture at a temperature greater than or
equal to about 2100.degree. F. for between about 20 hours and about 32
hours; and
lowering the temperature of the repair mixture to about 250.degree. F.
4. The method of claim 3 wherein the deposited repair mixture is
maintaining at a temperature between about 2000.degree. F. and about bout
2100.degree. F. for about 2 hours.
5. The method of claim 2, wherein the long term diffusion heat treatment
includes:
heating said mixture deposited on said damaged region to a temperature of
greater than or equal to about 2000.degree. F.;
decreasing the temperature of the deposited repair mixture from about
2000.degree. F. to a temperature between about 1999.degree. F. and about
1600.degree. F.;
maintaining the deposited repair mixture at a temperature between about
1999.degree. C. and about 1600.degree. F. for less than about 20 hours;
and
lowering the temperature of the repair mixture to about 250.degree. F.
6. The method of claim 5 wherein the deposited repair mixture is maintain
at a temperature greater than or equal to about 2000.degree. F. for about
2 hours.
7. The method of claim 1, wherein said joining includes:
heating the deposited repair mixture to a first temperature of between
about 800.degree. F. and about 1800.degree. F.;
heating the deposited repair mixture to a second temperature greater than
or equal to about 1800.degree. F.;
heating the deposited repair mixture to a third temperature between about
1800.degree. F. and less than about the incipient melting temperature of
the superalloy substrate for between about 15 and about 45 minutes; and
cooling the depositing repair mixture to a temperature of less than or
equal to about 1800.degree. F.
8. The method of claim 7 wherein the deposited repair mixture is maintained
at a temperature between about 800.degree. F. and about 1800.degree. F.
for about 15 minutes.
9. The method of claim 1, wherein the repair mixture comprises at least one
additional base metal alloy composition.
10. The method of claim 9, wherein said at least one additional brase alloy
composition is not a eutectic alloy.
11. The method of claim 1, and further including:
applying an environmental coating to said to said superalloy.
12. The method of claim 1 wherein said joining includes heating the
deposited repair mixture to a temperature of between about 800.degree. F.
and less than the incipient melting temperature of the superalloy.
13. The method of claim 1, wherein said joining includes heating the
deposited repair mixture under an inert atmosphere or under subatmospheric
conditions.
14. The method of claim 1 wherein said joining includes heating the
deposited repair mixture to a temperature sufficient to form a solid
solution matrix.
15. The method according to claim 1, wherein the cobalt-base braze alloy
composition is prealloyed, and wherein the base metal alloy composition is
prealloyed.
16. The method of claim 1, wherein the cobalt-base braze alloy composition
of consists essentially of, in percent by weight:
Nickel from about 9.5% to about 11.5%,
Chromium from about 22% to about 24%,
Aluminum from about 0.5% to about 2.5%,
Titanium from about 0.75% to about 2.25%,
Tungsten from about 2% to about 4%,
Platinum up to about 40%,
Palladium up to about 40%,
Rhenium from about 0.001% to about 2%,
Rare earth/Lanthanide series addition up to about 5%,
Tantalum from about 5% to about 7%,
Carbon up to about 1.05%,
Boron from about 0.5% to about 2.5%, and
Silicon from about 4% to about 6; and
the balance cobalt.
17. The method of claim 1, wherein the cobalt base braze composition
consists essentially of, in percent by weight:
Nickel from about 9% to about 11%,
Chromium from about 21.5% to about 23.5%,
Titanium from about 0.001% to about 0.25%,
Tungsten from about 6% to about 8%,
Rhenium from about 0.001% to about 15%,
Tantalum from about 2.5% to about 15%,
Platinum up to about 40%,
Palladium up to about 40%,
Rare earth/Lanthanide series addition up to about 5%,
Carbon up to about 1.1%,
Boron from about 0.5% to about 2.5%, and
Silicon from about 4% to about 6,
Zirconium from about 0.01% to about 1.5%; and
the balance cobalt.
18. The method of claim 1, wherein the cobalt base braze composition
consists essentially of, in percent by weight:
Nickel from about 29% to about 32%,
Chromium from about 13.75% to about 15.75%,
Aluminum from about 2.3% to about 4.4%,
Tungsten from about 0.3% to about 2.4%,
Rhenium from about 0.001% to about 1.5%,
Tantalum from about 7.8% to about 9.8%,
Hafnium from about 0.001% to about 1.5%,
Rare earth/Lanthanide series addition up to about 5%,
Platinum up to about 40%,
Palladium from about 2% to about 4%,
Carbon up to about 0.8%,
Boron from about 1.3% to about 3.4%, and
Silicon from about 2.3% to about 4.4,
the balance cobalt.
19. The method of claim 1, wherein the repair mixture is provided as a
powder metal slurry.
20. The method of claim 1, wherein the repair mixture is provided as a
pre-sintered powdered metal alloy tape.
21. The method of claim 1, wherein the repair mixture is provided as a
plasticized powdered metal alloy tape.
22. The method of claim 1, wherein the repair mixture is provided as a
pre-sintered alloy preform.
23. The method of claim 1, wherein the repair mixture comprises less than
or equal to about 50%, by weight based on the total weight of the repair
mixture, of the cobalt-base braze alloy composition.
24. The method of claim 1, wherein the repair mixture comprises less than
or equal to about 30%, by weight based on the total weight of the repair
mixture, of the cobalt base braze alloy composition.
25. A method of repairing a damaged region of a corrosion resistant
superalloy substrate comprising:
preparing a repair mixture comprising a cobalt-base braze alloy
composition, a base metal alloy composition; and an organic binder;
wherein the cobalt-base braze alloy composition is a eutectic alloy
comprising, in percent by weight;
Nickel from about 0.001% to less than the weight percent of cobalt;
at least one element selected from the group consisting of:
Rhenium from about 0.001% to about 15%,
Palladium from about 0.001% to about 40%,
Platinum from about 0.001% to about 40%,
Iridium from about 0.002% to about 12%, and
Ruthenium from about 0.001% to about 12%; and
the balance Cobalt;
depositing said mixture on said damaged region of said corrosion resistant
superalloy substrate; and
joining the repair mixture to the superalloy substrate.
26. A method of repairing a superalloy article, said method comprising a
long term diffusion heat treatment of a superalloy mixture of at least one
braze alloy and at least one base metal alloy comprising:
providing a solid superalloy mixture of at least one braze alloy and at
least one base metal alloy, said solid superalloy mixture including an
amount of brittle phases;
heating said solid superalloy mixture to a temperature of at least about
2000.degree. F.;
increasing the temperature of the solid superalloy mixture to a temperature
between about 2100.degree. F. and a temperature less than the incipient
melting temperature of solid superalloy;
maintaining the temperature of the solid superalloy mixture at a
temperature between about 2100.degree. F. and a temperature less than the
incipient melting temperature of the solid superalloy for between about 20
and about 32 hours; and
decreasing the temperature of the solid superalloy mixture to a temperature
less than or equal to about 250.degree. F.;
wherein said method homogenizes the solid superalloy mixture and reduces
the amount of brittle phases.
27. The method of claim 26 wherein the superalloy mixture is maintained at
a temperature of at least about 2000.degree. F. for about 2 hours.
28. The method of claim 26 wherein said providing comprises a high
temperature brazing cycle prior to long term diffusion treatment, said
high temperature brazing cycle comprising:
heating a superalloy mixture to a temperature between about 1800.degree. F.
and less than the incipient melting temperature of the superalloy article
to be repaired and thereafter decreasing the temperature of the superalloy
mixture to a temperature less than the incipient temperature of the
article being repaired to about 1800.degree. F. to provide the solid
superalloy mixture.
29. The method of claim 28 wherein the superalloy mixture is heated in the
high temperature brazing cycle to a temperature of between about 1.degree.
F. and about 400.degree. F. higher than in the long term diffusion
treatment.
Description
FIELD OF THE INVENTION
This invention relates generally to diffusion braze repair of superalloy
articles and more particularly to cobalt-base braze alloy compositions
containing at least one of the following elements: rhenium, palladium,
platinum, ruthenium, iridium; and to long term diffusion heat treatment of
repaired superalloy articles.
BACKGROUND OF THE INVENTION
High temperature operating environments such as those present in gas
turbine engines, power generation turbines, refinery equipment, and heat
exchangers demand parts composed of a variety of cobalt-, iron-, and
nickel-base metals known as superalloys. These superalloys are capable of
withstanding extremely high temperatures for extended periods of time, but
the extremely stressful temperature conditions to which superalloy
articles are subjected eventually take their toll upon the metal in a
number of ways.
The main types of damage to a superalloy article are cracks from thermal
fatigue, wide gap cracks, foreign object impact damage, and dimensional
reduction from mechanical wear. Because the cost of these superalloy
components is quite high, there is considerable incentive to repair these
types of defects rather than to scrap the part and replace it with a new
one. The high cost of these components, as well as the fact that
superalloy components, once damaged, tend to fail repeatedly in the same
region, also makes it critical that any repairs made have mechanical,
environmental, and processing properties equivalent to or better than the
original superalloy base metal.
Traditional methods for repairing damaged superalloy articles involve
choosing or creating an alloyed combination of elements that will melt at
a temperature below the melting temperature of the superalloy substrate.
These compositions are known in the industry as braze alloys, and the most
useful prior art braze alloys are characterized as either nickel-base or
cobalt-base alloys. Historically, the most popular braze alloys contain a
melting point depressant such as silicon or boron; a complex of some of
the same alloying elements used in the superalloy article to be repaired
such as chromium, aluminum, titanium, tungsten, etc.; and either nickel or
cobalt as the base. In fact, one braze alloy, sometimes known as B-28, is
simply the combination of an alloy frequently used to manufacture cast
turbine airfoils, named Rene '80, with about 2% boron.
Advances in the braze alloy composition art have introduced
multi-constituent alloy compositions that are mixtures of at least one
braze alloy and at least one base metal alloy, the base metal alloy
differing from the braze alloy in that it melts at a higher temperature
than the braze alloy and contains no melting point depressants that can
weaken the repair site. These multi-constituent compositions result in
stronger repairs because the low-melting brazing alloy liquefies first,
wetting the base metal constituent and joining the entire mixture to the
superalloy article.
Once a braze alloy or alloy mixture has been chosen, the damaged superalloy
article is cleaned to remove any environmental coating that may be over
the base metal and any oxides that may have developed inside the damaged
regions. The braze alloy composition is then applied to the region to be
repaired, and the article subjected to a high temperature brazing cycle to
melt and join the braze alloy to the superalloy article. Upon the
completion of this cycle, typical braze alloys will have formed
undesirable large blocky or script-like brittle phases composed of
chromium, titanium, and the family of refractory elements (e.g., tungsten,
tantalum) combined with the melting point depressants. These brittle
phases weaken the repair composite and cannot be removed from conventional
braze alloys.
However, certain braze alloy compositions, known as diffusion braze alloys,
are capable of withstanding higher temperatures than conventional braze
alloys. Diffusion braze alloys form the same bad phases during brazing as
conventional alloys, but diffusion braze alloys can be subjected to a
second, long-term high temperature heat cycle known as a diffusion cycle.
This diffusion cycle allows the brittle borides, carbides, and silicides
to break down into fine, discrete blocky phases. The diffusion cycle also
diffuses the elemental melting point depressants into the braze alloy
matrix. These actions result in a stronger repair that is less susceptible
to incipient melting when the part is returned to service.
Unfortunately, the diffusion braze alloys of the prior art have failed to
attain the crucial part-like mechanical and environmental properties
demanded by the increased stresses to which today's superalloy articles
are subjected. The main reason for this failure is that prior high
temperature braze alloys and alloy powder mixtures tend to use only those
elements present in the superalloy article being repaired.
This lack of flexibility in the compositions of the prior art has caused a
stagnation in the development of truly new braze alloy compositions which
employ elements and elemental combinations without regard to the
composition of the superalloy substrate. As well, previous
multi-constituent alloy compositions were so precisely matched to the
particular superalloy to be repaired that it was considered unthinkable to
select base metal powders for the mixture based solely on their mechanical
and environmental properties.
For these reasons, prior art compositions cannot provide a flexible
diffusion braze alloy system capable of accommodating various new elements
and base metal powders to increase the strength, flow characteristics, and
oxidation resistance of the braze alloy system. Prior art heat treatment
cycles are similarly incapable of effectively breaking down brittle phases
and allowing the elemental melting point depressants to diffuse both into
the superalloy substrate and the base metal matrix. As well, prior art
diffusion braze alloy compositions frequently rely upon intentional carbon
additions for strength, and these prior art compositions do not
effectively impart improved environmental resistance to the superalloy
substrate and/or any environmental coating which may be applied to the
substrate.
A need therefore exists for a new diffusion braze alloy system that
desirably employs the elements rhenium, platinum, palladium, ruthenium,
iridium, and/or aluminum in order to improve significantly over the hot
corrosion and oxidation resistance properties provided by prior art braze
alloys. Additionally, such an improved braze alloy composition preferably
uses boron and silicon concurrently as melting point depressants in order
to reduce the undesirable mechanical and environmental properties
associated with the use of either boron or silicon alone. The present
invention addresses these needs.
SUMMARY OF THE INVENTION
Briefly describing one aspect of the present invention, there is provided
an improved cobalt-base braze alloy composition and method for diffusion
braze repair of superalloy articles that achieves mechanical, processing,
and environmental properties equivalent to and, in many cases, better than
those properties possessed by the superalloy articles. The present
cobalt-base braze alloy composition comprises nickel; at least one element
selected from the following group: rhenium, palladium, platinum,
ruthenium, iridium; boron; silicon; and cobalt. This composition may also
include one or more of the rare earth elements such as yttrium, cerium,
lanthanum, and other lanthanide series elements; aluminum; chromium;
titanium; tungsten; molybdenum; niobium; hafnium; tantalum; iron;
manganese; and/or zirconium, which elements appear in many advanced
superalloy base metal compositions. This cobalt-base braze alloy
composition may be combined with one or more powdered base metal
superalloy compositions to form an improved diffusion braze alloy mixture
having enhanced mechanical, environmental, and processing properties
compared to prior art braze alloy mixtures. The present invention also
provides new cobalt-base base metal alloy compositions for use in such
improved diffusion braze alloy mixtures, which base metal alloy
compositions do not include melting point depressants but which are
otherwise similar to those of the braze alloy compositions.
In the case of non-eutectic alloys according to the present invention, the
instant invention employs melting point depressants such as boron,
silicon, and aluminum to reduce the melting point of the braze alloy.
Although the present braze alloy compositions contain relatively low
amounts of melting point depressants, these depressants nonetheless
adversely affect the mechanical and/or environmental properties of a
repaired article unless they are subjected to a long-term diffusion heat
treatment cycle.
The present invention therefore also describes an improved diffusion heat
treatment method to break down the undesirable phases formed by the
melting point depressant(s) and diffuse the depressant(s) into the base
metal alloy matrix. Use of this long-term diffusion heat treatment method
minimizes the negative properties associated with the use of conventional
melting point depressants.
In the brazing method of the present invention, a damaged region of a
superalloy article is repaired by first cleaning the article by any
conventional means; preparing a braze alloy composition mixture according
to the present invention, wherein the mechanical and environmental
properties of that mixture are chosen to equal and preferably improve upon
those properties of the superalloy article to be repaired; depositing this
mixture on the region to be repaired; and placing the superalloy article
in a furnace under an inert gas atmosphere or under a vacuum. Once in such
a furnace, the pressure in the furnace chamber should be reduced to
approximately 1.times.10.sup.-1 torr or a lower pressure and the brazing
cycle initiated by heating the repaired region to a temperature of about
800.degree. F. The 800.degree. F. temperature is maintained for
approximately 15 minutes, whereafter the temperature is increased to about
1800.degree. F. and that temperature maintained for approximately 15
minutes. Next, the temperature is again raised to a temperature less than
the incipient melting temperature of the article being repaired, which
incipient melting temperature typically exceeds 2350.degree. F., and that
less than incipient melting temperature maintained for between 15 and 45
minutes. Finally, the furnace is vacuum cooled from the less than
incipient melting temperature to about 1800.degree. F. This step completes
the conventional brazing cycle which causes the formation of undesirable
brittle phases. The next steps in the present method constitute the
diffusion heat treatment cycle that will break down these brittle phases.
Upon completion of the high temperature brazing cycle, the superalloy
article is subjected to a pressure higher than the pressure used in the
brazing cycle and reheated to a temperature of between 1 and 400.degree.
F. below the chosen brazing temperature for the article. This temperature
is maintained for at least 20 hours, whereafter the temperature is lowered
to about 250.degree. F. At this point, the superalloy article is fully
repaired and ready for machining.
The superalloy article is then usually coated with a metal or ceramic,
diffusion or overlay coating according to any known application method.
This coating protects the superalloy base metal from oxidation and hot
corrosion attack, and, if the superalloy article is given a multi-layer
coating of which at least one layer is a cobalt-base braze alloy according
to the present invention, the coating remains resistant to environmental
attack much longer than a traditional coating.
These and other objects, advantages, and features are accomplished
according to the compositions and methods of the following description of
the preferred embodiment of the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
For the purposes of promoting an understanding of the principles of the
invention, reference will now be made to the preferred embodiments
thereof, and specific language will be used to describe the same. It will
nevertheless be understood that no limitation of the scope of the
invention is thereby intended, such alterations, modifications, and
further applications of the principles of the invention being contemplated
as would normally occur to one skilled in the art to which the invention
relates.
The principal objective of the present invention is to achieve mechanical,
processing, and environmental resistance properties in a braze alloy
repair composite that equal, if not exceed, the levels of these properties
enjoyed by the superalloy substrate, or base metal. Prior art braze alloy
compositions have failed to achieve this objective for several reasons.
First, prior art braze alloy systems are unable to reduce the melting
point of the brazing alloy without embrittling either the repair composite
or the superalloy substrate. Second, skilled artisans have for so long
considered it crucial that the elements of the braze alloy system match
those of the superalloy article to be repaired that it was thought
impossible or unworkable to repair a superalloy article using any other
elements. And lastly, this old way of thinking about braze alloy
compositions prevented artisans from investigating what base metal alloys
could be added to a braze alloy system to improve a repair's solid
solution strengthening and oxidation resistance properties.
It is clear that braze alloy systems having the foregoing problems cannot
effectively repair today's higher temperature and higher strength
superalloys which undergo greater mechanical and thermal stresses than
ever before, and which cost more to manufacture than ever before.
Therefore, the compositions and method of the present invention introduce
new elements and elemental combinations not previously considered for use
in the field of diffusion braze repair in order to overcome the
disadvantages of the prior art and to give the superalloy component a
longer useful life than was previously considered attainable.
The cobalt-base diffusion braze alloy composition of the present invention
has the following general composition range, by weight:
Elements Weight Percent
Cobalt Balance
Nickel 0.001-<Co
Chromium 0-40
Aluminum 0-12
Titanium 0-6
Tungsten 0-15
Molybdenum 0-15
Niobium 0-12
Ruthenium 0.001-12
Rheniun 0.001-15
Iridium 0.001-12
Hafnium 0-6
Tantalum 0-15
Platinum 0.001-40
Palladium 0.001-40
Iron 0-3
Manganese 0-1
Rare Earth/Lanthanide 0-5
Series Element(s)
Carbon 0-2
Boron 0.001-6
Silicon 0.001-10
Zirconium 0-2
While the foregoing constitutes a general description of the cobalt-base
braze alloy composition of the present invention, the following are
specific examples of preferred compositions according to the present
invention. These specific examples are provided for purposes of
illustrating the invention, and no limitations on the invention are
intended thereby. Before proceeding further, it should also be noted that
when zeroes are used in the composition tables of the present invention,
those zeroes indicate no intentional addition of the element, not that the
element is absent from the composition. It is usually not economically
feasible to use 100% pure elemental additions, and therefore some
impurities may be introduced into the composition.
A first preferred embodiment of the cobalt-base braze alloy composition of
the present invention is known as "RCA-C1" and has the following
composition:
Elements Weight Percent
Cobalt Balance
Nickel 10.5
Chromium 23
Aluminum 1.5
Titanium 1.75
Tungsten 3
Rhenium 1
Tantalum 6
Platinum 0-40
Palladium 0-40
Carbon 0-0.55
Boron 1.5
Silicon 5
A second preferred embodiment of the cobalt-base braze alloy composition of
the present invention is known as "RCA-C2" and has the following
composition:
Elements Weight Percent
Cobalt Balance
Nickel 10
Chromium 22.5
Titanium 0.1
Tungsten 7
Rhenium 0.001-15
Tantalum 3.5
Platinum 0-40
Palladium 0-40
Carbon 0-0.6
Boron 1.5
Silicon 5
Zirconium 0.5
A third preferred embodiment of the cobalt-base braze alloy composition of
the present invention is known as "RCA-C3" and has the following
composition:
Elements Weight Percent
Cobalt Balance
Nickel 10.5
Chromium 20.5
Aluminum 2.25
Tungsten 1.25
Rhenium 1
Tantalum 7.75
Platinum 0-40
Palladium 0-40
Carbon 0-0.28
Boron 3
A fourth preferred embodiment of the cobalt-base braze alloy composition of
the present invention is known as "RCA-C6" and has the following
composition:
Elements Weight Percent
Cobalt Balance
Nickel 30.75
Chromium 14.75
Aluminum 3.38
Tungsten 1.38
Rhenium 0.5
Tantalum 8.8
Hafnium 0.5
Platinum 0-40
Palladium 3
Carbon 0-0.3
Boron 2.33
Silicon 3.38
A most preferred embodiment of the cobalt-base braze alloy composition of
the present invention is known as "RCA-C4" and has the following
composition:
Elements Weight Percent
Cobalt Balance
Nickel 10.5
Chromium 23
Aluminum 1.75
Tungsten 1.25
Rhenium 1
Platinum 0-40
Palladium 0-40
Tantalum 6.5
Carbon 0-0.55
Boron 2.15
Silicon 3.25
Turning now to discuss the novelty of the foregoing compositions, it will
be obvious to one of ordinary skill that certain preferred embodiments of
the instant diffusion braze alloy compositions are formulated with
concurrent boron and silicon additions as melting point depressants. Prior
art braze alloys, in contrast, have traditionally used boron alone as the
melting point depressant for two major reasons: (1) boron diffuses
exceptionally well into the base metal matrix of a braze alloy mixture,
and (2) this boron diffusion results in a higher remelt temperature of the
final repair composite. Boron in a braze alloy thus ensures that the
repair composite will be able to withstand the same high temperatures
withstood by the superalloy substrate itself.
However, exceptionally high boron concentrations in a braze alloy promote
embrittlement of the superalloy base metal and incipient melting. These
deleterious effects reduce the number of repairs that can be performed
upon any one region of a superalloy article and thereby shorten the
operating life of the part since superalloy components tend to fail
repeatedly in the same area.
Silicon alone is typically used in conventional (non-diffusion) brazing
alloy compositions to speed the alloy's rate of flow into a damaged
region. Unfortunately, silicon-only braze alloys do not typically have a
high degree of diffusivity into the base metal matrix, and they tend to
form very stable silicides. These silicides form large, brittle,
script-like phases in the microstructure of the repair composite, which
phases can degrade the mechanical properties of both the repair composite
and the superalloy base metal.
Embodiments of the present invention combine the two elements to minimize
the undesirable effects of using either boron or silicon alone and
maximize the beneficial properties imparted by each element. When boron
and silicon are combined, the amount of boron necessary to reduce the
melting temperature of the alloy is decreased, which reduces the high
concentrations of boron in the superalloy substrate. The instant braze
alloy system thus enjoys the strength and high temperature melting
properties imparted by boron without having to sacrifice the superalloy
base metal in the process.
Similarly, the silicon additions in the present braze alloy compositions
improve the flow characteristics of the braze alloy without embrittling
the repair composite with large amounts of script-like silicide phases.
This latter benefit is assured when the long-term diffusion heat treatment
cycle of the present invention is used to homogenize the braze alloy/base
metal mixture and diffuse the elemental boron and silicon into the base
metal matrix. Silicon also has the unexpected benefit of improving the
performance of any environmental coating placed over the repaired region.
This feature helps assure long life of the repaired area and gives it
improved environmental resistance properties over the original superalloy
substrate.
It should be understood, however, that the use of either boron or silicon
alone as a melting point depressant is also considered and intended to
come within the scope of the present invention. As will be discussed in
greater detail below, the use of iridium, ruthenium, palladium, platinum,
and especially rhenium in the preferred compositions of the present
invention significantly reduce the deleterious brittle phases associated
with the use of boron alone and thereby help to increase the re-melt
temperature of the final repair composite. The present compositions,
therefore, achieve unexpected results over traditional boron- or
silicon-only diffusion braze alloys.
Cobalt-base base metal alloy compositions are also intended to come within
the scope of the present invention. As discussed previously, braze alloy
compositions may be used alone to repair part damage, but significant
benefits in mechanical strength and processing properties can be achieved
when a part is repaired using a mixture of one or more braze alloys and
one or more base metal components. The main reason for these improvements
over single-component braze alloy systems is that the amount of melting
point depressants used can be significantly reduced. To achieve such
property improvements, then, the present invention has described braze
alloy compositions which may be combined with any known superalloy base
metal to create an improved repair composite. The following discussion
describes new base metal alloy compositions that can be combined with the
instant and/or any other known braze alloy compositions to also create an
improved repair composite.
The base metal alloy compositions described herein possess the same general
composition range as the braze alloy compositions of the present
invention, but obviously do not include boron or silicon. Therefore, the
instant base metal alloy compositions comprise generally, by weight:
Elements Weight Percent
Cobalt Balance
Nickel 0.001-<Co
Chromium 0-40
Aluminum 0-12
Titanium 0-6
Tungsten 0-15
Molybdenum 0-15
Niobium 0-12
Ruthenium 0.001-12
Rhenium 0.001-15
Iridium 0.001-12
Hafnium 0-6
Tantalum 0-15
Platinum 0.001-40
Palladium 0.001-40
Iron 0-3
Manganese 0-1
Rare Earth/Lanthanide 0-5
Series Element(s)
Carbon 0-2
Zirconium 0-2
While the foregoing constitutes a general description of the cobalt-base
base metal alloy composition of the present invention, the following are
specific examples of preferred compositions according to the present
invention. These specific examples are provided for purposes of
illustrating the invention, and no limitations on the invention are
intended thereby.
A first preferred embodiment of the cobalt-base base metal alloy
composition of the present invention is known as "RCA-B1" and has the
following composition:
Elements Weight Percent
Cobalt Balance
Nickel 10
Chromium 22.5
Aluminum 2
Tungsten 5
Rhenium 0.5
Tantalum 6
Platinum 0-40
Palladium 0-40
Carbon 0-0.55
Zirconium 0.5
A most preferred embodiment of the cobalt-base base metal alloy composition
of the present invention is known as "RCA-B2" and has the following
composition:
Elements Weight Percent
Cobalt Balance
Nickel 10.5
Chromium 22
Aluminum 1.75
Tungsten 4
Tantalum 6.5
Rhenium 0-15
Palladium 0-40
Platinum 0.001-40
Carbon 0-0.55
As can be seen in all the foregoing diffusion alloy compositions, braze
alloys and base metal alloys alike, the instant alloy compositions
contemplate use of one or more elements from the following group: rhenium,
palladium, platinum, ruthenium, and iridium. The use of rhenium,
palladium, platinum, ruthenium, and iridium in cobalt-base diffusion
alloys represents a significant advance in the art of diffusion braze
repair of superalloy articles because it departs radically from the
traditional diffusion braze alloy composition: a powder of the same
superalloy as the damaged component with a measure of melting point
depressants added to lower the brazing temperature. These new alloy
compositions are formulated to not only repair, but also to improve, the
mechanical, processing, and environmental properties possessed by the
superalloy base metal.
It is well known that failures in superalloy components regularly occur in
the same or an immediately adjacent location. It is therefore extremely
important that these areas of fatigue be repaired to be even stronger than
the original superalloy base metal. The compositions of the present
invention achieve this objective by successfully combining certain
elements such as rhenium, platinum group elements, aluminum, and one or
more rare earth/lanthanide series elements, and by removing carbon from
the compositions.
The first of these new preferred elements, rhenium, is preferably added to
the cobalt-base alloy compositions of the present invention in an amount
from 0 to 15 weight percent. Rhenium additions give the present
compositions significantly improved mechanical and environmental
properties over other, more traditional, solid solution strengthening
elements such as tungsten, molybdenum, or hafnium. The mechanical
properties associated with rhenium compositions are similar to those
achievable by using tungsten and molybdenum; however, rhenium has
significant oxidation resistance properties that the tungsten and
molybdenum-type elements do not have. Therefore, the inclusion of rhenium
in the compositions of the present invention permits a skilled artisan to
reduce or completely remove other solid solution strengthening elements
that are undesirable for use in oxidizing environments. Of additional
benefit to the preferred composition embodiments, rhenium does not promote
sigma phase formation in the repair composite or the adjacent superalloy
base metal.
Another benefit of rhenium-containing compositions according to the present
invention relates to rhenium's effect on melting point depressants in the
alloy matrix. Unexpectedly, the addition of rhenium to the present
preferred braze alloy compositions works so well to bind up significant
amounts of melting point depressants that the elements which traditionally
form brittle phases (e.g., chromium, tungsten) are left in solid solution
to strengthen the repair composite and improve environmental resistance.
As well, the instant preferred compositions eliminate the diffusion of
excess melting point depressants into the adjacent base metal of the
superalloy article. This is true even when silicon is not used
concurrently with boron, and the amount of melting point depressants that
can be successfully incorporated in the alloy matrix increases with the
length of the long-term heat treatment diffusion cycle. The present
compositions can therefore use boron alone to lower the melting
temperature of the braze alloy and achieve the benefit of a higher re-melt
temperature for the repair composite without experiencing the weak and
destructive brittle phases or the excess boron diffusion experienced with
the prior art boron-containing braze alloys.
Platinum may be added to the present compositions in a range of from 0 to
40 weight percent. The addition of platinum and/or other platinum group
elements, such as ruthenium, osmium, rhodium, iridium, and palladium,
improves the hot corrosion and oxidation resistance properties of the
repair composite. As well, platinum and other platinum group metals added
to the present invention in sufficiently high concentrations improve the
ductility, or plasticity, of the repair composite.
The addition of palladium is contemplated by the present invention because
it achieves improvements in the repair composite similar to those achieved
by platinum. For example, palladium enhances the oxidation resistance of
the repair site and improves the ductility of the repair composite.
Palladium also enhances the flow characteristics of the instant braze
alloy compositions, and nickel and palladium are 100% soluble when
combined in a braze alloy mixture. Further, palladium additions have been
shown to retard the formation of undesirable borides and silicides in the
alloy matrix.
The addition of ruthenium in the compositions of the present invention
improves the repair composite in ways similar to those discussed above for
the other platinum group additions. Ruthenium additions are also
beneficial in that they reduce alloy density while simultaneously
providing strength equivalent to or better than that achieved by the
foregoing elemental additions. This strength characteristic is especially
beneficial given that ruthenium's atomic weight is 30% less th an other
similar refractory elements commonly used in cobalt superalloys. An
increase in the strength to weight ratio presents a significant benefit to
the aerospace industry because a lighter structure having strength equal
to or greater than a structure formed from more traditional, heavy
materials may be formed using the compositions of the present invention.
Another element contemplated by and intended to come within the scope of
the present invention is aluminum. Conventional high temperature braze
alloys such as AMS 4783 do not have aluminum in them. This is because
aluminum reduces the flowability of the braze by the rapid formation of
aluminum oxide, a material commonly used for the prevention of braze flow.
Additionally, different surface tensions and viscosities occur that change
the braze flow characteristics when aluminum is used. Because diffusion
braze alloys do not have the same flow requirements a s conventional braze
alloys, diffusion braze alloys allow the use of aluminum. Nonetheless,
aluminum is not normally used in cobalt-base superalloy repair because
prior art repair systems typically use powdered cobalt superalloys
combined with a braze alloy to repair a cobalt superalloy substrate, and
cobalt superalloys do not contain aluminum.
Cobalt superalloys are typically used in the temperature range at which the
superalloy base metal is subject to hot corrosion attack and damage.
Certain turbine manufacturers have recently begun to push the operating
temperatures for cobalt superalloys above this temperature range and into
the oxidation mode of base metal attack and damage. It is for this reason
that the present invention includes aluminum in a cobalt-base diffusion
braze alloy composition. By including aluminum in the instant
compositions, the final repair composite receives additional protection
from preferential oxidation at the repaired areas of the superalloy
components; the gamma prime phase of the alloy matrix is strengthened over
non-aluminum containing cobalt-base braze alloys; and the introduction of
aluminum helps reduce the melting point of the braze alloy composition.
These benefits outweigh any previously encountered difficulties with braze
flow characteristics, and the inclusion of aluminum represents a
significant advance in the diffusion braze alloy art.
The present invention also contemplates and intends that the preferred
embodiments of the present invention incorporate one or more rare earth
elements such as yttrium, cerium, lanthanum, and other lanthanide series
elements. This addition so significantly improves the unexpected and novel
oxidation resistance enjoyed by the braze alloy and the base metal alloy
compositions of the present invention and the brazeability of the instant
braze alloy compositions that the amounts of aluminum, boron, and silicon
used in these compositions may be reduced. By reducing the aluminum
content, any problems that might arise from alumina formation during
brazing can be minimized. The reduction of boron and silicon additions
permits the properties of the repair composite to more closely resemble
the properties of the base metal substrate, and the reduction of boron
yields additional oxidation resistance in the present braze alloy
compositions.
It is well known in the art that, other than using solid solution
strengthening elements, carbides are the primary strengthening mechanism
for cobalt-base alloys. Because the compositions of the present invention
include such effective solid solution strengthening elements as rhenium,
and because the present compositions contemplate the use of suicides
and/or borides to strengthen the alloy matrix as effectively as carbides,
carbon may effectively be removed from the present compositions without
suffering any loss in mechanical properties.
It is particularly beneficial to remove carbon from diffusion alloy
compositions because carbon prefers to agglomerate and precipitate out of
the alloy matrix at lower temperatures. Carbides therefore exhibit poor
ductility and have poor oxidation resistance. Carbide particles in a
cobalt-base alloy system also tend to go into solution in the alloy matrix
and disappear at high temperatures. However, as soon as the superalloy
cools, the carbides precipitate out of the matrix and form a carbide line
at the interface of the repair composite and the superalloy substrate.
This carbide line allows the repair composite to break away from the
superalloy substrate in a zipper-like fashion. The mere possibility of
such a significant repair failure makes removing carbon from the present
invention a significant improvement in the art.
Of importance, the most preferred embodiments of the present compositions
are prealloyed powders. The prealloying is accomplished using well-known
methods according to the following procedure: the basic elements are first
mixed in the required weight percentages in a container; this mixture is
then melted at high temperature; and the molten mixture is atomized by
spraying the metal through a high pressure nozzle and cooling it with
argon gas. This technique solidifies the once discrete elements into
uniform powder particles. Skilled artisans will recognize that the
properties of a prealloyed mixture are significantly different from those
of a simple mixture of elements, and the improvements achieved by the
present invention rely in part upon the fact that these compositions are
prealloyed.
The present alloy compositions contemplate the inclusion of a number of
other elements typically used in advanced superalloy compositions,
including solid solution strengtheners such as cobalt, molybdenum, and
tungsten; gamma-prime formers such as nickel, hafnium, niobium, titanium,
and tantalum; sacrificial oxide formers such as chromium; carbide formers
such as zirconium; elements to improve ductility such as manganese; and
other elements such as iron. Because these elements are commonly used in
superalloy base metals and braze alloys and because the properties they
impart to those systems are well known in the art, those of ordinary skill
will understand which elements to choose to customize the instant
compositions to their specifications.
Having now described the preferred composition formulations of the present
invention, it is necessary to discuss the preferred mixtures for use in
repairing a damaged superalloy component. It is known in the art of
superalloy repair that combining in a braze alloy mixture a high
temperature melting composition and one or more compositions which melt at
a lower temperature will improve the strength of the repair composite
while still providing adequate flow characteristics to facilitate
placement and insertion of the braze alloy system into the damaged region.
However, the high temperature component used in prior mixtures was nothing
more than a powder of the same superalloy as the article being repaired.
The present invention, in contrast, describes a diffusion braze alloy
system that employs base metal powders chosen without regard to the
composition of the superalloy substrate. Instead, the present invention
chooses which base metal powders to incorporate based on the properties
those base metals will impart to the braze alloy system or, the repair
composite. In certain preferred embodiments of the present invention, the
use of multiple base metal components, whether iron-, cobalt-, or
nickel-base, enhances the mechanical, environmental, and processing
properties of the instant braze alloy system.
As an example, one base metal powder may be chosen for its strength and
another base metal powder chosen for its improved braze flow
characteristics. One preferred embodiment of the mixture of the present
invention uses a base metal alloy powder known in the industry as
Mar-M509. Mar-M509 is known to provide a very strong repair composite, but
it is not preferred for use in diffusion braze repair because it slows the
flow of molten braze mixture during the high temperature braze cycle. This
slow flow characteristic is especially undesirable when the damage to the
superalloy article is in the form of a crack or a wide gap. It is
therefore desirable when repairing cracks and gaps to include a second
base metal powder known in the industry as X40. When used alone, X40 makes
for a relatively weak repair composite, but when combined with Mar-M509,
it improves the flow characteristics of the braze alloy system and permits
cracks and gaps to be filled with a stronger repair composite. Certain
other preferred embodiments of the present invention choose the high
temperature base metal alloy compositions of the present invention in
order to impart the improved properties associated with those base metal
powders to the braze alloy mixture.
Although the following may generally be known in the industry, it is
instructive for practicing the present invention that in the embodiments
of the present braze alloy mixtures preferred for repairing cracks, the
braze alloy composition or compositions comprise no more than 50% by
weight of the total braze alloy mixture. Wide cracks and gaps may be
repaired with the present mixtures if the percentage by weight of the
braze alloy composition or compositions is kept to about 40%. Similarly,
dimensional repairs, or build-ups, are most effectively performed when the
total weight of braze alloy in the mixture does not exceed 40%.
It will be obvious to those of ordinary skill which mixture percentages
should be applied to which types of structural damage. Accordingly, one
preferred embodiment of the braze alloy mixture of the present invention
comprises a powder metal slurry. Another preferred embodiment of the
present mixture invention takes the form of a plasticized powdered metal
alloy tape. Another preferred embodiment of this mixture comprises a
pre-sintered alloy tape. Alternatively, one preferred embodiment of the
present invention especially useful for dimensional repair comprises a
pre-sintered alloy preform.
In practice, after the damage has been assessed, the preferred braze alloy
composition or compositions of the present invention chosen, the base
metal alloy composition or compositions chosen, and the braze alloy and
base metal compositions combined in the appropriate ratio corresponding to
the damage to be repaired, the superalloy article is cleaned of all
coatings and oxides using techniques known in the art for such cleaning.
The chosen braze alloy mixture in the embodiment appropriate to repair the
damage, e.g., powder metal slurry, tape, etc., is then applied to the
damaged region and the superalloy article subjected to a high temperature
brazing cycle in a vacuum or in an inert gas atmosphere. This high
temperature brazing cycle melts the braze alloy portion of the mixture,
thereby creating a base metal powder matrix within the braze alloy
composition, and joining the entire mixture to the now-repaired superalloy
substrate.
One preferred inventive method for repairing damaged superalloy components
involves a high temperature brazing cycle having the following steps:
placing the mixture-coated superalloy article in an inert gas atmosphere
or under vacuum in a brazing furnace; obtaining a pressure of
1.times.10.sup.-3 torr or lower pressure in the inert gas atmosphere or
under the vacuum; heating the braze alloy mixture to a temperature of
about 800.degree. F. and holding that temperature for approximately 15
minutes; thereafter increasing the temperature to about 1800.degree. F.
and holding that temperature for approximately 15 minutes; then increasing
the temperature again to less than the incipient melting temperature of
the article being repaired, which incipient melting temperature typically
exceeds 2350.degree. F., and holding that less than incipient melting
temperature for between 15 and 45 minutes; whereafter the furnace is
vacuum cooled from the less than incipient melting temperature to about
1800.degree. F.
While the foregoing high temperature braze cycle has been described, it
will be understood by skilled artisans that any series of temperatures and
brazing times capable of melting only the braze alloy composition and
permitting that braze alloy composition sufficient time to flow and effect
the repair while forming a solid solution matrix and precipitating
gamma-prime phase particles are considered and intended to be encompassed
herein. Those of ordinary skill in the art will also understand that the
lower the pressure in the brazing furnace during this brazing cycle, the
lower the vapor pressure of the sacrificial oxide forming elements, and
thus the better the flow of the braze alloy during the braze cycle.
The next series of steps in the preferred repair method of the present
invention comprise the long term diffusion heat treatment cycle. This
diffusion cycle is critical to homogenize the remaining solidified braze
alloy system microstructure and diffuse the elemental melting point
depressants into the alloy matrix. The particular steps used in this
diffusion heat treatment cycle comprise the following: obtaining a
pressure in the furnace higher than the pressure used in the high
temperature braze cycle, preferably in the range of about 250 torr;
heating the mixture deposited on the repaired region to a temperature of
about 2000.degree. F.; holding the temperature at about 2000.degree. F.
for approximately 2 hours; increasing the temperature to about
2100.degree. F.; holding the temperature at about 2100.degree. F. for
approximately 22 hours; and lowering the temperature from about
2100.degree. F. to about 250.degree. F.
While this diffusion cycle may be altered slightly in terms of the
temperatures employed, the range of preferred temperatures for the
diffusion braze cycle of the present invention are between 1.degree. and
400.degree. F. less than the highest temperature achieved during the high
temperature brazing cycle. The range of preferred pressures includes any
pressure higher than the pressure used in the braze cycle but lower than
atmospheric pressure. Those of ordinary skill will recognize that the
higher the pressure, the less chromium and other elemental vaporization
from the repair composite and the superalloy article there will be, and
therefore the less elemental loss there will be.
Additionally, the diffusion braze holding times may vary slightly from the
holding times described above, but preferred holding times are in the
range of at least 20 hours to about 32 hours in order to permit the repair
composite sufficient time to break down the script-like silicide phases
into fine discrete particles. Preferred diffusion cycle times are also
adequate both to reduce the size and quantity of brittle boride phases in
the repair matrix caused by chromium, titanium, and members of the
refractory family of elements (tungsten, tantalum, etc.) combining with
boron, and to diffuse the elemental boron and silicon into the repair
composite matrix.
Upon completion of the long term diffusion heat treatment cycle, the
repaired part is usually given a new metal or ceramic, diffusion or
overlay coating by means of any known coating method. Such coatings
protect both the superalloy article and/or the repaired area from
oxidation, hot corrosion, and extreme thermal gradients. Examples of
typical environmental coatings are simple aluminides, platinum aluminides,
MCrAl(X)-type overlays, and ceramics. Typical metal coatings such as these
may be used alone as a single layer coating, as the final layer of a
multilayer coating, or as a bonding coat for a ceramic top coat; and the
ceramic coatings may be used alone directly atop the superalloy article
surface, or as the final coating atop a bonding coat. However, it is also
contemplated by and intended to come within the scope of the present
invention to use the present cobalt-base braze alloy compositions as a new
type of metal coating that may be used to coat a superalloy article by
means of any coating method. The instant compositions may also form part
of a multilayer coating system in which the present compositions are
applied to the surface of a superalloy article either before or after
another environmental coating has been applied.
It has been discovered through the course of high temperature cyclic
oxidation testing of superalloy parts coated and/or repaired according to
the present invention that the combination of the present braze alloy
composition(s) with one or more environmental coatings yields unexpected,
inventive, and beneficial improvements in oxidation resistance.
Specifically, the instant cobalt-base braze alloy compositions
significantly improve the adhesion of an environmental coating to the
repair composite.
By way of example and not of limitation, the cyclic oxidation testing was
performed at both 2075.degree. F. and 2000.degree. F. on repaired cobalt
base metal coupon specimens, and the specimens of both test conditions
exhibited similar results. The test performed at 2075.degree. F. indicated
that the coating spalled off of the cobalt base metal specimens after 40
cycles. The coating did not spall off the braze repaired areas of the
coupons, but it was consumed after 300 cycles. The coating around the
brazed areas started to spall after approximately 100 cycles. At
2000.degree. F., the test results were identical, except the coating over
the repair composite lasted over 500 cycles with no loss of coating. It is
believed that these surprising achievements in oxidation resistance are a
result of the careful balance struck between the oxidation properties and
the mechanical properties of the elements used in the present preferred
compositions.
While the invention has been described in detail in the foregoing
description, the same is to be considered as illustrative and not
restrictive in character, it being understood that only the preferred
embodiments have been shown and described, and that all changes and
modifications that come within the spirit of the invention are desired to
be protected.
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